1. Field of the Invention
The present invention relates to an ultrasonic injection mold for an optical disk. In particular, the present invention relates to an ultrasonic injection mold for an optical disk that has a good transferability and that can effectively prevent the optical disk from generating birefringence and warpage.
2. Description of the Prior Art
Optical disks such as CD""s and DVD""s are molded by injecting a resin such as melted polycarbonate into a mold. The mold comprises a movable die and a fixed die. A circular cavity is formed on the contacting surfaces of the movable die and fixed die for molding a disk. Further, on the bottom of the cavity, a stamper is positioned for forming convexes and concaves (a pit).
When a thin product such as an optical disk mentioned above is molded by injection, it is necessary to prevent a non-uniform filling by improving flowability of the melted resin being filled into the cavity so that defective products with shrinkage deformation and/or birefringence will not be manufactured. In order to prevent such defective products, in accordance with the injection molding method disclosed in Japanese Patent Laid-Open Application No. Hei10-661, the relationship between the equivalent diameter of the mold (D) and the amplitude of the ultrasonic oscillation (xcex) is kept as D less than 0.6xcex while the injection molding is performed.
In the prior art ultrasonic molding method, however, since only the longitudinal oscillation is used, oscillation to be transferred in the radial direction is generated when the diameter of the surface where a cavity is formed so as to mold an optical disk (contacting surface of the fixed die and the movable die), which is usually 210 mm, is more than a half of the resonant wavelength of the ultrasonic, which is more than 130 mm at the frequency of 19 KHz in the case of SUS type mold. Therefore, it is impossible for the fixed portion of the mold and the mounted portion of the nozzle to be matched to the xe2x80x9cnodexe2x80x9d of the oscillation (where the amplitude becomes zero) and it is difficult to maintain the mold and nozzle.
Also since only the longitudinal oscillation is used in the structure of the prior art ultrasonic mold, the whole mold comprising the movable die and fixed die needs to be at least one wavelength thick. Thus, the total thickness exceeds 400 mm when the oscillation direction converter is included, which makes it impossible to position this mold between the die plates of the production device (30 ton Injection molding device).
The present invention aims to provide an ultrasonic injection mold for injection molding an optical disk that will minimize the amplitude of the ultrasonic to be transferred in the radial direction of the cavity so that this mold will have a good transferability and will keep the mold itself and nozzle in appropriate states between the die plates while effectively preventing the generation of birefringence and shrinkage deformation.
The ultrasonic injection mold for an optical disk in accordance with the present invention comprises: a movable die; a fixed die; a cavity for molding a disk formed on the contacting surfaces of the movable die and fixed die; a stamper positioned at the bottom of the cavity for forming a pit on a surface of the disk; and an ultrasonic generation means provided on the movable die or the fixed die for applying ultrasonic to the cavity in performing infection molding, wherein one or more slits are formed in the regions of the movable die and fixed die in which the cavity is formed, the one or more slits being oriented in the direction which crosses the radial direction of the cavity.
Since the one or more slits make thinner the area where they are formed, the oscillation of that area is suppressed, thereby decreasing direction of the cavity.
The one or more slits are preferably formed at the distance of nxcex/4 (n=1, 3, 5, . . . ) from the ultrasonic input portion where the ultrasonic is input into the movable die or fixed die, wherein xcex is the wavelength of the ultrasonic the transmitted direction of which is to be converted within the movable die or the fixed die. The xe2x80x9cultrasonic input portionxe2x80x9d means to be such portions where the ultrasonic generation means or oscillation direction converter and the movable die or fixed die are contacted.
The amplitude of resonant oscillation becomes a node at nxcex/4 (n=1, 3, 5, . . . ). The amplitude of the oscillation to be transferred in the radial direction will be kept to the minimum by positioning each of the slits at these nodes.
The ultrasonic generation means preferably generates oscillations throughout the whole bodies of the fixed and movable dies in the lateral direction or in the radial direction to be transmitted to the radial direction of the cavity as well as in the longitudinal direction. Thus, the thickness of each of the movable die and fixed die will be at least halved as compared to the case only the longitudinal oscillation is employed.
Further, a punch means may be provided on the movable die and oscillation direction converter or on the fixed die for punching an injection molded product at a predetermined position. This punch means may construct a product model means for extruding the product.
Furthermore, a supporting member is preferably inserted into the one or more slits so that the bottom of the one or more slits may be supported by one end of the supporting member in performing injection molding. In such an instance, it is preferable to keep t in the range of 0.1 less than t less than 0.3 mm where t is the distance between the supporting member and the internal wall of the one or more slits.